1. Field of the Invention
This invention relates to a method of reducing the NOx content in the waste gases leaving the regenerators of a cross-fired regenerative furnace used for melting glass. NOx is a shorthand designation of oxides of nitrogen such as NO and NO.sub.2.
2. Discussion of the Prior Art
It has long been known that a fuel burner operating substoichiometrically (i.e. at an air:fuel ratio less than that necessary to effect complete combustion) will produce less NOx than when operating with stoichiometric conditions, and burners designed to operate in that manner are described e.g. in U.S. Pat. No. 4,878,830 which also reviews the prior art in this field. JP-A-53-8361 (Examined 48134/84) describes a method of operating a glass furnace using afterburners, to introduce additional fuel into the furnace in the vicinity of a port, regenerator, heat exchange chamber or flue. U.S. Pat. No. 4,347,072 discusses this specification and points to problems in operating a glass furnace in the manner described in JP-A-55-8361. U.S. Pat. No. 4,347,072 describes an alternative method of operating by supplying hydrocarbons into the waste gases from fuel combustion above the glass melt and then burning this excess fuel in the furnace to provide heat energy to the melting process.
It has always been considered that operating a glass furnace with the melting conditions reducing, i.e. substoichiometric, would produce glass of poor quality.
U.S. Pat. No. 4,559,100 in the name of the major glass maker PPG describes a process where the conditions in the vicinity of the melting glass are prevented from becoming substoichiometric so as to avoid producing poor quality glass. The process requires that additional fuel should be injected into the melting chamber at a flow rate and volume sufficient to provide an O.sub.2 rich region above the glass and a fuel rich region thereabove, and to further provide relatively low overall excess air and at least substantially complete combustion by the time the combustion gases exit the melting chamber. Substoichiometric conditions clearly have occurred by chance from time to time in glass tanks and, as they have resulted in poor glass, have directed people away from operating continuously with reducing conditions in the furnace.
We have now found that a reduction in the amount of NOx in the waste gases leaving a flue system of a glass melting tank can be achieved by ensuring that the waste gases leaving the furnace and entering the regenerator includes fuel which has not undergone complete combustion. All of the previous proposals to operate with non-stoichiometric conditions are concerned with the melting chamber, and ensuring that oxidizing conditions are maintained within the melting chamber at all times and where excess fuel is supplied, ensuring that it is burnt before it enters the regenerator system, or that as the fuel passes through the regenerator that conditions are consistently oxidizing. Our invention is based on the discovery that it is possible to minimize the amount of NOx in the exit flue gases from a regenerative glass melting tank by ensuring that there are combustibles present in the waste gases as they pass through the regenerators. This combustible material is a mixture of unburnt fuel, combustible material produced by the effect of heat on the fuel and other radicals produced in this pyrolysis. A part of this material is capable of reacting with NOx in the waste gases and converting it to harmless material. It is essential to operate with a sealed regenerator so that the ingress of air into the regenerators is such as to avoid uncontrolled combustion within the refractory packing or checkerwork structure, which reduces the effectiveness of the process of removal of NOx from the waste gases. In particular, the burners are sealed into the burner block/port neck refractories of the regenerators. It is ensured that there is no excess air in the checkerwork structure which would cause uncontrolled combustion of the fuel within the checkerwork structure which would damage the structure due to overheating. The combustible material is burnt by adding air preferably after it has left the checkerwork structure of the regenerator, or at points within the checkerwork, dependent on the temperature regime within the regenerator system.